The present invention relates to a method and apparatus for controlled feeding of sheets from a stack of sheets into a printing or another sheet processing or consuming machine. More particularly, the invention relates to a method and apparatus for feeding sheets of paper or the like in such a way that each and every failure of the sheet feeding mechanism to remove from the stack a predetermined number of sheets (normally a single sheet) at a time results in the generation of signals which can be used to eliminate the cause of malfunctioning and/or for other purposes.
Unsatisfactory feeding of sheets is attributable to malfunctioning of the sheet feeding mechanism, to exhaustion of the supply of sheets which form a stack, to improper stacking of sheets, and/or to a tendency of sheets in the stack to adhere to each other with a force that cannot be overcome by the suction cups or analogous sheet lifting or removing members of the feeding mechanism.
It is well known that faulty operation of the sheet feeding mechanism in or for a printing or like machine can cause serious problems. Such problems arise when the feeding mechanism fails to deliver a sheet as well as when the feeding mechanism delivers more than a desired number of sheets. For example, the printing machine is likely to dispense ink into or onto its parts when no sheet is held in proper position for reception of ink, and the surplus sheets are likely to clog the printing machine and or cause damage to or even a breakage of its parts. Therefore, the surplus sheets should be intercepted prior to their entry into the processing machine.
It is already known to combine a sheet feeding mechanism with various control devices which monitor the operation of the sheet feeding mechanism and produce signals in response to detected absence of sheets and/or in response to detected presence of more than a desired number of sheets. In accordance with a presently known proposal, the sheet material which is removed by the feeding mechanism is scanned by a monitoring device which is capable of detecting the presence of two or more sheets and unlocks a rotary segment which closes a switch to thereby generate a signal for transmission to the processing machine. A drawback of this monitoring device is that it is reliable only while the sheets are being fed at a relatively low rate; when such rate is exceeded, the parts of the monitoring device begin to vibrate and are likely to indicate the presence of two or more sheets even if the feed of sheets is entirely satisfactory. Another drawback of this monitoring device is that its operation is too slow because its parts must scan each removed sheet individually. Moreover, the just described mode of monitoring is likely to compound the errors in scanning of individual sheets, and the generation of a signal in response to detection of more than a desired number of sheets is delayed to such an extent that one or more additional sheets are likely to enter the processing machine before the latter is arrested or otherwise influenced in response to the generation of a signal.
It was further proposed to combine a sheet feeding mechanism with a system of suction cups serving to separate from each other those sheets which are removed by the feeding mechanism whenever the mechanism removes more than a single sheet. Such system failed to gain widespread acceptance in the industry because the separation of sheets which adhere to each consumes much time and also because the separating action of suction cups is not sufficiently reliable, especially if the feeding mechanism has removed three or more sheets.